Mooring anchor

ABSTRACT

A dynamic mooring anchor of the pivoted fluke type is disclosed which includes an elongated shank, a stock connected transversely to the aft end of said shank and a fluke pivotally connected to the aft end of the shank to pivot below the shank between a first minimum angular position and a second maximum angular position relative to the shank. The shank, fluke and stock have surfaces which are substantially free of obstructions to the flow of bottom soil thereover.

United States Patent 11 1 11] 3,766,877 Beck Oct. 23, 1973 [54] MOORING ANCHOR 2,591,236 4/1952 Christie 114/208 R 709,914 9/1902 Lenox [75] Inventor: Beck Hmlston, 2,674,969 4/1954 Danforth 114 207 [73] Assignee: Esso Production Research Company,

Houston, Tex, Primary Examiner-Duane A. Reger Assistant ExaminerGa1en L. Barefoot [22] Flled' Sept 1970 Attorney-James A. Reilly, John B. Davidson, Lewis [21] Appl. No.: 73,798 H. Eatherton, James E. Gilchrist, Robert L. Graham and James E. Reed [52] U.S. Cl 114/208 R [51] Int. Cl B63b 21/40 [57] ABSTRACT [58] Field of Search 114/208 R, 2'07, 206 R A dynam1c mooring anchor of the pivoted fluke type [56] References Cited is disclosed which includes an elongated shank, a UNITED STATES PATENTS stock connected transversely to the aft end of said shank and a fluke pivotally connected to the aft end of l1/:962 Towne et al. 114/208 R the shank to pivot below the Shank between a first 3022762 g minimum angular position and a second maximum anl55045l 8/1925 Pardey 47208 R gular position relative to the shank. The shank, fluke 493:901 3/1893 Starr 1 14 R and stock have surfaces which are substantially free of 1 373 742 4 1921 Jones 4 2 R obstructions to the flow of bottom soil thereover. 2,743,695 5/1956 Bowman 114/208 R X 21 Cl 5 D 2,869,503 1 1959 Winslow 114/208 R raw'ng gums 2,981,219 4/1961 Winslow... 114/208 R 3,618,554 11/1971 Patten 114/208 R FIG. 2

INVE VTOR. ROBERT W. BECK ATTORNEY PATENTED URI 2 3 I915 SHEET 2 [If 2 I "ENTOR.

. ECK

BY 75w fW ROBERT ATTORNEY BACKGROUND OF THE INVENTION 7 1. Field of the Invention This invention relates to an improved mooring anchor for use by floating drilling vessels and other vessels which must be securely moored for extended periods of time. 3 l

2. Description of the Prior Art Anchors may generally be classed in one of three groups; weight anchors, which rest on the bottom and rely primarily on weight and friction to' generate holding power; semidynamic anchors, which partially bury in the sea floor when dragged, thereby utilizingboth weight and soil resistance for development of holding power; and dynamic anchors, which bury themselves completely in the bottom soil and rely almost entirely on the reaction of thesoil against the fluke of the anchor for development of holding power. The present invention is directed to an improved anchor of the latter type which is particularly useful in mooring floating drilling vessels and other vessels which must be securely moored at one location for extended periods of time. l Dynamic anchors may be further subdivided into fixed fluke anchors and pivoted fluke anchors. The former usually having a single fluke and are normally set by means of two lines the mooring line and a pendant. A significant disadvantage of fixed fluke anchors stems from the fixed angle described between the shank and the fluke; The inability to change this angle results in the holding power developed by the anchor varying substantially, depending on the nature and quality of bottom sediments. It is known, for instance, that the fluke angle should be limited to 3035 for relatively firm sand'bottoms, whereas greater fluke angles, ranging up to 50, are superior. for softer mud or clay bottoms. Thus, the fluke angle selected'either will be optimum for one bottom condition but not the other or will represent a compromise between the two.

Pivo ted fluke anchors normally have two flukes that pivot freely to either side of the shank, which are intended to permit them to dig in regardless of which side of the fluke comes in contact with the sea floor. These anchors can be lowered and set with a single line. In addition, pivoted fluke anchors may readily be adapted for use as variable fluke angle anchors. By means of a wedge or similar device the fluke angle can be adjusted at the mooring site to maximize holding power developed by the anchor. under the bottom conditions encountered.

Unfortunately, despite these advantages, pivoted fluke anchors, like fixed fluke anchors, have not proven entirely satisfactory for mooring floating drilling vessels. They frequently drag along the ocean floor without tripping'their flukes and'under these, conditions are unable to dig in and develop any substantial holding power. The additionof tripping palms and similar devices to the flukes for the purpose of forcing them to dig into the bottom has alleviated this problem to some extent, but has not greatly increased their reliability as mooring anchors. For example, these anchors will often dig in and develop substantial holding power only to suddenly, and heretofore inexplicably, give way and completely pull out. To counter this erratic performance of mooring anchors, either additional mooring lines have been employed, or extra anchors have been set in tandem on each line. These practices, while providing some relief, are both time consuming and expensive and therefore leave a great deal to be desired.

SUMMARY OF THE INVENTION Anchor drag tests carried out with both model and full size anchors have led to the discovery that certain structural features of mooring anchors have caused their past erratic performance. In particular, it has been found that obstructions to the flow of bottom soil over the anchor seriously impair development of any substantial sustained level of holding power. These flow impediments compactsediments lying in front of the anchor along its drag path. Compacted material builds up as the anchor advances and eventually causes the anchor to ball up and frequently to pull out of the bottom. In mud or clay bottoms, accumulations of material are trapped on the leading surfaces and tops of the flukes and between the shank and fluke assembly, particularly at the intersection thereof, or where stiffening ribs, tripping palms or other flow obstructions exist. In sand bottoms, sediments do not adhere to the anchor but gather ahead of the anchor and compact into stagnant masses which have the same detrimental affect.

It has been found that if an anchor is constructed in accordance with the present invention, these problems will be greatly alleviated. The present invention is directed to an improved mooring anchor which offers very slight resistance to the flow of bottom sediments thereover, permitting it to bury deeply and develop and sustain substantial holding power.

Broadly, the improved dynamic mooring anchor of the invention comprises an elongated shank adapted to be supported at its fore and aft ends and a stock connected transversely to the aft end thereof. A fluke is pivotally connected to the aft end of the shank and is positioned below the shank to pivot freely between a first minimum angular position and a second maximum angularposition below and relative to the shank. The flow surfaces of the shank, flukeand stock are adapted to be substantially free of obstructions to the flow of bottom soil thereover. Thefluke may be adapted to permit adjustment of the maximum angle between the fluke andthe shank. I t

The significantly improved performance of the anchor of the invention is attributable to a number of structural features. The anchor has no crown or tripping palms, either of which can trap stagnant masses of soil when the anchor is dragged. By bevelingthe lower edge of the shank and the leading edge of the fluke, providing the-fluke with a smooth upper surface free from ribs or other obstructions, and adapting the connection between ,the shank and the fluke to present very slight resistance to flow of bottom sediments, the overall flow resistance of the anchor is sharply reduced.

This prevents compaction and balling up of sediments lying in front of the anchor along its drag path, allowing the anchor to bury itself deeply and to develop and sustain substantial holding power. The pivoted fluke which opens only in a downward direction provides a number of advantages. Although, unlike other pivoted fluke anchors, the anchor of the invention will usually be set by two lines, this procedure facilitates controlling the orientation of the anchor, thereby insuring that the anchor lands right side up, i.e., with the fluke beneath the shank. Because the pivoted fluke is adapted to form a minimum angle with the shank even when it is in a closed position, the fluke will rapidly dig into the bottom when the anchor is dragged. The need for tripping palms or other flow obstructions otherwise necessary to impart a positive digging action to the flukes is thereby eliminated. This in turn permits a substantial reduction in the flow resistance of the anchor. Moreover, adapting the pivoted fluke to remain on the lower side of the shank also permits use of a single fluke, which is inherently stronger than the bifurcated flukes normal to pivoted fluke anchors and is more economical to construct. In addition, the pivoted fluke permits provision of a means for adjusting fluke angle so that it may be tailored to the bottom conditions encountered. It also permits the anchor to fold, resulting in substantial space savings during stowage aboard the vessel. In view of these advantages and others which will become apparent from the detailed description and drawings that follow, it will be clear that the anchor of the invention represents a significant improvement over anchors known heretofore.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view depicting the top of an anchor constructed in accordance with the invention.

FIG. 2 is a side elevation view, partially in section, of the anchor of FIG. 1.

FIG. 3 shows a plan view, partially in section, of the bottom of the anchor of FIG. 1.

FIG. 4 is a cross section of the shank taken along line 4-4 in FIG. 3.

FIG. 5 is a perspective view of the anchor of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the drawing, the anchor of the present invention includes an elongated shank 11 having its leading edge, i.e., the edge which cuts through the bottom soil, sharply beveled to reduce resistance of the shank to penetration through the bottom material. The degree of beveling of the shank is best shown in the cross-sectional view of the shank, FIG. 4. In a preferred embodiment of the invention the shank is beveled so that the leading edge forms an included angle not greater than 45 It will be apparent, however, that the acute angle selected may be varied substantially so long as it is consistent with maintenance of shank strength and reduction of flowresistance to bottom materials thereover. Means'for attaching an anchor line near the leading end of the shank is also included, e.g., shackle 13. The opposite end of the shank may suitably form a Y with the lower branch 15 having a hole 17 therethrough for receiving the stock 19. The upper branch 21 of the Y may be adapted to cooperate with means such as wedge 23 to control the maximum angle described between fluke 25 and the shank. While the configuration shown is preferred, it will be apparent that the aft end of the shank can be pivotally connected to the fluke in other fashions as well.

A single fluke 25 is shown having a slot 27 adapted to-permit lower branch 15 of the Y formed at the aft end of the shank to extend therethrough. It is important that the upper surface of the fluke, i.e., the flow surface over which bottom sediments-pass as the anchor is dragged forward, be a smooth, streamlined surface free from ribs, protruding crowns, or other flow obstructions. The leading edge of the fluke should preferably be sharply beveled to facilitate its entrance into the bottom sediments. This beveling may best be. seen in FIGS. 2 and 3. Resistance to penetration of the fluke is also reduced by tapering the front end into one or more sharp points as shown. The connection between the shank and the fluke is adapted to present very slight resistance to the flow of bottom sediments thereby.

The fluke may include means for attaching a pendant line, such as pad eye 29 shown attached to the trailing end of the fluke. This means could alternatively be attached to the aft end of the shank. One or more strengthening ribs may be attached to the lower surface of the fluke. Locating them beneath the fluke is advantageous as this removes them from the flow path of the bottom soil. While a box 31 enclosing four ribs 33 is shown in the drawing, other configurations of support members may be utilized. The degree of strengthening needed will of course be determined by the area of the erture in ribs 33 and thenthrough hole 17 in lower branch 15 of the Y-shaped aft end of the stock. The stockmay be constructed with hollow ends rather than of bar stock throughout ifit is desired to reduce weight. In the position shown the stock serves as a connecting pin, permitting the shank and fluke to pivot with respect to one another. The length of the stock should be sufficient to provide lateral stability to the anchor when it penetrates the sediments. The stock should also be long enough so that if the anchor contacts the bottom in a tilted manner, the anchor will be supported on the forward end of the shank, one end of the stock and one tip of the fluke. If the stock is too short the anchor may be unstable and fall over rather than penetrate the bottom sediments. In this connection it will be noted that the anchor of the drawing includes two fluke tips which are somewhat, displaced from one another. This arrangement facilitates engagement of the fluke in the bottom when the anchor is tilted.

A means is providedforlimiting the minimum acute angle between the pivoted fluke and the lower edge of the shank to some angle greater thanzero. This forces the anchor to remain partially open at all times. The means employed may be a keyslot such as 27 in fluke 25 which causes the fluke to engage the shank at an angle of about 21. It will be obvious that other means could also be employed to limit the minimum angle between the fluke and shank to some angle greater than zero.

A means is also provided for limiting the maximum acute angle between the pivoted fluke and the lower edge of theshank. Wedge 33- is shown positioned on the fluke and situated so as to cooperate with upper branch 21 at the aft end of the shank. This wedge controls the maximum angle described between the shank and the fluke, and by varying the size of the wedge run with the anchor the fluke angle can be controlled to optimize the holding power the anchor develops in a particular bottom. 1

To employ the anchor of the invention to moor a floating drilling vessel or other structure, it is desirable to first ascertain bottom conditions. These may be established either by sampling or by other well known techniques such as conducting tests with small anchors. Once the nature and quality of the bottom sediments are known, the fluke angle may be adjusted to optimize holding power developed under the particular conditions encountered. Optimum. holding power will be developed by the anchor of the invention with a fluke angle of about 34 for sand bottoms and 50 for mud or clay. The anchor is normally set by means of an auxiliary work boat equipped with a winch. The mooring line which is connected to the vessel to be moored is attached to shackle 13 at the lead end of the shank. The pendant line from the work boats winch is connected to pad eye 29 on the aft end of the anchor, normally on the fluke. a

The anchor is then lowered to bottom while tension is maintained in each of the lines. While it is suspended, the anchor will orient itself so that the heavy fluke hangs downward. Lowering is continued until the anchor comes into contact with the ocean bottom positioned with the fluke down. The pendant line is then preferably tied off and attached to a buoy for ease of retrieval. As the mooring line is pulled, the anchor which is already positioned with its fluke down and open to the extent of the minimum fluke angle quickly penetrates the bottom sediments. The reaction of the soil against the fluke causes the fluke angle to open up to the optimum angle selected for peak holding power. When it is desired to terminate the mooring operation, the pendant line is retrieved and a winch aboard an auxiliary work boat is employed to hoist the anchor aboard.

Scale models of the anchor of the invention were extensively tested in comparison with models of prior art anchors of the pivoted fluke type which were built to the same scale. These tests were carried out in model tanks, in beach sand and at various offshore locations and resulted in holding power data representative of a wide variety of bottom conditions. Throughout the tests performance of the prior art anchors was impaired primarily by two types of difficulties (1) flukes failing to pivot and diginto the bottom soil, and (2) anchors balling upand subsequentlypulling out of the bottom soil. In contrast, the anchor of the invention demonstrated a high degree of reliability and its comparative performance was superior to that of the prior art anchors tested, as for example, the prior art anchors disclosed in U.S. Pat. Nos. 2,677,343 and 3,015,299.

The following'table compares the performance of an 800 lb anchor constructed in accordance with the invention with an 800 lb prior art anchor of the type disclosed in US. Pat. No. 2,677,343, which has pivoted flukes and a crown projection to assure positive fluke action. The data were developed from a series of anchor drag tests run at three different locations in the Anchor pull tests were carried out with a boat equipped with a tensionometer. The pull tests tabulated were categorized in three different classes: (1) fully successful and pulled out, and (3) unsuccessful indicating the anchor developed less than 15,000 lbs holding power. The anchor of the invention was able to develop high holding power ranging from 25,000-60,000 lbs on all tests and it never balled up or lost holding power, even when dragged from 600900 feet. The prior art anchor, on the other hand, was only able to achieve moderate to high holding power during 50 percent of the drags, and it balled up and lost holding power in half of those cases. It was fully successful in only 25 percent of the total drag tests run.

What is claimed is:

l. A dynamic mooring anchor of the pivoted fluke type comprising an elongated shank; a stock connected transversely to the aft end of said shank; a single fluke having a smooth upper surface positioned below said shank and having its aft end pivotally connected to the aft end of the shank; means for limiting the minimum acute angle described between said fluke and the underside of said shank; means at the aft end of said shank for limiting the maximum acute angle described be tween said fluke and the underside of said shank; said shank, fluke and stock having flow surfaces substan tially free of obstructions to passage of bottom soil thereover.

2. Apparatus defined by claim 1 wherein said fluke is a single fluke.

3. Apparatus defined by claim I including adjustable means for controlling the maximum angle described between said fluke and said shank.

-4. Apparatus defined by claim .3 wherein said maximum angle is within a range between 30 and 50.

5. Apparatus defined by claim 4 wherein said minimum angle between said fluke and said shank is about 6. A dynamic mooring anchor of the pivoted fluke type comprising an elongated shank having a fore end and an aft end; a stock connected transversely to the aft end of said shank; means for connecting an anchor line to the fore end of said shank; a fluke having a smooth upper surface positioned below said shank and having its aft end pivotally connected to the aft end of the shank; said shank, fluke and stock and the connection therebetween having surfaces substantially free of obstructions to the flow of bottom soil thereover; means for limiting the minimum acute angle described between said fluke and the underside of said shank, and means situated at the aft end of said shank and cooperating .with said fluke and said shank for limiting the maximum acute angle described between said fluke and the underside of said shank; said means cooperating to limit the angular motion of said fluke to an arc situated beneath said shank and defined between said minimum acute angle and said maximum acute angle. a 7. Apparatus defined by claim 6 wherein said fluke is bifurcated.

8. Apparatus defined by claim 6 including adjustable means for controlling the maximum angle described between said fluke and said shank.

9. Apparatus defined by claim 8 wherein said maximum angle is within a range between 30 and 50.

10. Apparatus defined by claim 9 wherein said minimum angle between said fluke and said shank is about 11. A dynamic mooring anchor of the pivoted fluke type comprising an elongated shank; a stock connected transversely to the aft end of said shank; a single fluke having a smooth upper surface having its aft end pivotally connected to the aft end of said shank; means for limiting the minimum acute angle described between said fluke and the underside of said shank; and means at the aft end of the shank and cooperating with said fluke and said shank for limiting the maximum acute angle described between said fluke and the underside of said shank; said means cooperating to limit angular movement of said fluke to an are described between minimum and maximum acute angular positions of said fluke below and relative to said shank.

12. Apparatus defined by claim 11 wherein said fluke is bifurcated.

13. Apparatus defined by claim 11 including adjustable means for controlling the maximum angle described between said fiuke and said shank.

14. Apparatus defined by claim 13 wherein said maximum angle is within a range between 30 and 50.

15. Apparatus defined by claim 14 wherein said minimum angle between said fluke and said shank is about 21.

16. A dynamic mooring anchor having an elongated shank provided with means near one end for attaching a mooring line, a stock extending perpendicular to the longitudinal axis of the shank near the other end, a single fluke having a smooth upper surface pivotally connected to the shank near the stock for angular rotation beneathsaid shank about an axis parallel to the stock; means for limiting the minimum acute angle described between said fluke and the underside of said shank; and means situated at the aft end of the shank for limiting the maximum acute angle described between said fluke and the underside of said shank; said means cooperating to limit angular movement of said fluke between a first limiting position in which the fluke extends at a minimum acute angle from the shank and a second limiting position in which the fluke extends at a maximum angle from the shank.

17. Apparatus defined by claim 16 including'adjustable means for controlling the maximum angle described between said fluke and said shank.

18. A dynamic anchor of the pivoted fluke type comprising an elongated shank; a single elongated fluke having a'smooth upper surface and having a central,

longitudinal slot near the aft end thereof to permit the aft end of said shank to extend therethrough; a transverse stock pivotally connecting the end of the shank extending through the fluke to the underside of said fluke; means for limiting the minimum acute angle described between said fluke and the underside of said shank; and means at the aft end of the shank for limiting the maximum acute angle described between said fluke and the underside of said shank; said means cooperating to permit angular rotation of said fluke only on the underside of said shank and limited to an arc described between minimum and maximum acute angular positions of the fluke relative to the shank.

19. Apparatus defined by claim 18 including adjustable means for controlling the maximum angle described between said fluke and said shank.

20. Apparatus defined by claim 2 wherein said minimum angle between said fluke and said shank is about 21.

21. A dynamic anchor of the pivoted fluke type comprising:

a. an elongated shank having a sharply beveled lower edge and a Y-configured aft end, said Y having upper and lower branches and said lower branch having an aperture therethrough for receiving a stock, said shank being adapted to be supported at the fore end thereof;

b. a single elongated fluke having a smooth upper surface and a sharply beveled leading edge and having a central slot positioned on the longitudinal axis thereof near the aft end, said slot being adapted to permit the lower branch of said Y to extend therethrough.

. a plurality of longitudinalstiffening ribs affixed to the underside of said fluke, said ribs having apertures therethrough for receiving and attaching a stock thereto;

d. a transverse stock extending through said apertures in said stiffening ribs, and said aperture in said lower branch of said Y, said stock being rigidly connected to said fluke and journaled through said shank to permit angular rotation of said flukebeneath said shank; and

e. means for limiting the minimum acute angle described between said fluke and the underside of said shank; and means at the aft end of the shank and cooperating with the shank and the fluke for limiting the maximum acute angle described between said fluke and the underside of said shank. 

1. A dynamic mooring anchor of the pivoted fluke type comprising an elongated shank; a stock connected transversely to the aft end of said shank; a single fluke having a smooth upper surface positioned below said shank and having its aft end pivotally connected to the aft end of the shank; means for limiting the minimum acute angle described between said fluke and the underside of said shank; means at the aft end of said shank for limiting the maximum acute angle described between said fluke and the underside of said shank; said shank, fluke and stock having flow surfaces substantially free of obstructions to passage of bottom soil thereover.
 2. Apparatus defined by claim 1 wherein said fluke is bifurcated.
 3. Apparatus defined by claim 1 including adjustable means for controlling the maximum angle described between said fluke and said shank.
 4. Apparatus defined by claim 3 wherein said maximum angle is within a range between 30* and 50*.
 5. Apparatus defined by claim 4 wherein said minimum angle between said fluke and said shank is about 21*.
 6. A dynamic mooring anchor of the pivoted fluke type comprising an elongated shank having a fore end and an aft end; a stock connected transversely to the aft end of said shank; means for connecting an anchor line to the fore end of said shank; a fluke having a smooth upper surface positioned below said shank and having its aft end pivotally connected to the aft end of the shank; said shank, fluke and stock and the connection therebetween having surfaces substantially free of obstructions to the flow of bottom soil thereover; means for limiting the minimum acute angle described between said fluke and the underside of said shank, and means situated at the aft end of said shank and cooperating with said fluke and said shank for limiting the maximum acute angle described between said fluke and the underside of said shank; said means cooperating to limit the angular motion of said fluke to an arc situated beneath said shank and defined between said minimum acute angle and said maximum acute angle.
 7. Apparatus defined by claim 6 wherein said fluke is bifurcated.
 8. Apparatus defined by claim 6 including adjustable means for controlling the maximum angle described between said fluke and said shank.
 9. Apparatus defined by claim 8 wherein said maximum angle is within a range between 30* and 50*.
 10. Apparatus defined by claim 9 wherein said minimum angle between said fluke and said shank is about 21*.
 11. A dynamic mooring anchor of the pivoted fluke type comprising an elongated shank; a stock connected transversely to the aft end of said shank; a single fluke having a smooth upper surface having its aft end pivotally connected to the aft end of said shank; means for limiting the minimum acute angle described between said fluke and the underside of said shank; and means at the aft end of the shank and cooperating with said fluke and said shank for limiting the maximum acute angle described between said fluke and the underside of said shank; said means cooperating to limit angular movement of said fluke to an arc described between minimum and maximum acute angular positions of said fluke below and relative to said shank.
 12. Apparatus defined by claim 11 wherein said fluke is bifurcated.
 13. Apparatus defined by claim 11 including adjustable means for controlling the maximum angle described between said fluke and said shank.
 14. Apparatus defined by claim 13 wherein said maximum angle is within a range between 30* and 50*.
 15. Apparatus defined by claim 14 wherein said minimum angle between said fluke and said shank is about 21*.
 16. A dynamic mooring anchor having an elongated shank provided with means near one end for attaching a mooring line, a stock extending perpendicular to the longitudinal axis of the shank near the other end, a single fluke having a smooth upper surface pivotally connected to the shank near the stock for angular rotation beneath said shank about an axis parallel to the stock; means for limiting the minimum acute angle described between said fluke and the underside of said shank; and means situated at the aft end of the shank for limiting the maximum acute angle described between said fluke and the underside of said shank; said means cooperating to limit angular movement of said fluke between a first limiting position in which the fluke extends at a minimum acute angle from the shank and a second limiting position in which the fLuke extends at a maximum acute angle from the shank.
 17. Apparatus defined by claim 16 including adjustable means for controlling the maximum angle described between said fluke and said shank.
 18. A dynamic anchor of the pivoted fluke type comprising an elongated shank; a single elongated fluke having a smooth upper surface and having a central, longitudinal slot near the aft end thereof to permit the aft end of said shank to extend therethrough; a transverse stock pivotally connecting the end of the shank extending through the fluke to the underside of said fluke; means for limiting the minimum acute angle described between said fluke and the underside of said shank; and means at the aft end of the shank for limiting the maximum acute angle described between said fluke and the underside of said shank; said means cooperating to permit angular rotation of said fluke only on the underside of said shank and limited to an arc described between minimum and maximum acute angular positions of the fluke relative to the shank.
 19. Apparatus defined by claim 18 including adjustable means for controlling the maximum angle described between said fluke and said shank.
 20. Apparatus defined by claim 2 wherein said minimum angle between said fluke and said shank is about 21*.
 21. A dynamic anchor of the pivoted fluke type comprising: a. an elongated shank having a sharply beveled lower edge and a Y-configured aft end, said Y having upper and lower branches and said lower branch having an aperture therethrough for receiving a stock, said shank being adapted to be supported at the fore end thereof; b. a single elongated fluke having a smooth upper surface and a sharply beveled leading edge and having a central slot positioned on the longitudinal axis thereof near the aft end, said slot being adapted to permit the lower branch of said Y to extend therethrough; c. a plurality of longitudinal stiffening ribs affixed to the underside of said fluke, said ribs having apertures therethrough for receiving and attaching a stock thereto; d. a transverse stock extending through said apertures in said stiffening ribs, and said aperture in said lower branch of said Y, said stock being rigidly connected to said fluke and journaled through said shank to permit angular rotation of said fluke beneath said shank; and e. means for limiting the minimum acute angle described between said fluke and the underside of said shank; and means at the aft end of the shank and cooperating with the shank and the fluke for limiting the maximum acute angle described between said fluke and the underside of said shank. 